1. Field of the Invention
The present invention relates to a condenser for condensing exhaust steam from a steam turbine used in a thermal power plant or a nuclear power plant.
2. Description of the Related Art
A steam turbine used in a thermal power plant or a nuclear power plant sends the steam worked and expanded therein to a surface condenser. The exhaust steam flowing into this condenser is heat-exchanged with cooling water, such as seawater, river water, etc. in cooling tubes and condensed and collected.
FIG. 7 shows the schematic structure of a conventional condenser. In a condenser shell 1, two steam cooling tube bundles 2 of the same construction are disposed. Hereinafter, the description is made with respect to only one of steam cooling tube bundles 2, for the simplicity of the explanation. Steam cooling tube bundle 2 is composed of a number of steam cooling tubes 15 which are disposed in parallel with each other and extend horizontally. Steam cooling tube bundle 2 is divided into an upper tube bundle 2A and a lower tube bundle 2B. In a central space 16 formed between upper tube bundle 2A and lower tube bundle 2B, an enclosure 3 is disposed, in which an air cooling tube bundle 4 is disposed.
Air cooling tube bundle 4 is composed of a number of air cooling tubes 17 which are disposed in parallel with each other and extend in the extending direction of steam cooling tubes 15, and cools non-condensable gases, such as Air, Ammonia and the like which are contained in turbine exhaust steam or flow therein from other systems and parts. Further, at the lower part of condenser shell 1, a hot well 5 is disposed to collect and discharge drain (condensed water) condensed in steam cooling tube bundles 2 and air cooling tube bundles 4.
The turbine exhaust steam discharged from the steam turbine enters into condenser shell 1, and flows into steam cooling tube bundle 2 from the outer circumference of steam cooling tube bundle 2. Then the turbine exhaust steam is condensed on the surfaces of steam cooling tubes 15, while it flows toward air cooling tube bundle 4. The drain condensed in steam cooling tube bundle 2 drips into hot well 5.
The turbine exhaust steam contains Ammonia gas generated through decomposition of a corrosion inhibitor poured in boiler feed water. And therefore, with the condensation of the steam in steam cooling tube bundle 2, the concentration of Ammonia solved in the drain increases gradually. As partial pressure of steam drops on the surface of air cooling tubes 17 of air cooling tube bundle 4, steam is further condensed, and as a result, the drain (condensed water) of high Ammonia concentration is generated. The drain of high Ammonia concentration also drips into hot well 5 from air cooling tube bundle 4.
Further, Copper alloy is generally used for steam cooling tubes 15 of steam cooling tube bundle 2. This Copper alloy cooling tubes has the nature to be corroded severely with the drain (condensed water) of high Ammonia concentration. This phenomenon is called "Ammonia attack". Therefore, Titanium which has excellent corrosion resistance is generally used for air cooling tubes 17 of air cool tube bundle 4.
However, a conventional condenser has such a problem that steam cooling tubes 15 composing lower tube bundle 2B are corroded by the drain of high Ammonia concentration, because the drain of high Ammonia concentration condensed in air cooling tube bundle 4 drips into hot well 5 while contacting with steam cooling tubes 15 of lower tube bundle 2B. This kind of problem is also generated in a case where turbine exhaust steam contains corrosive gases other than Ammonia. Further, in many cases, with the progress of the condensation of steam, the corrosion is generated near support plates and tube plates where Ammonia of high concentration tends to be generated.